The cellulose ester film, particularly a cellulose triacetate film, is widely used for optical applications such as a polarizing plate, since film having sufficient transparency and a small anisotropy in refractive index can be easily prepared from the cellulose ester.
The attempt to prepare a polarizing plate having excellent scratch, chemical and glare resistance has been made by using an improved polyester protective film for a polarizing plate in which the protective film is applied with an iodine type or two color dye type polarizing film.
The liquid crystal displaying device employing such a polarizing plate does not require an extra protective film or plate for the polarizing plate, and therefore, a displaying device which is light, thin and highly discriminating can be obtained.
The liquid crystal displaying device is used for a displaying panel of calculators, watches, portable TV's, personal computers, and gauges for cars. As a displaying panel is increased, reduction of the thickness or weight of such a displaying device is an important object in addition to high quality image and full color image.
In conventional liquid crystal displaying devices, the displaying panel has a glass plate or a plastic plate on the surface, since the polarizing plate is poor in scratch and chemical resistance.
In Japanese Utility Model O.P.I. Publication No. 54-130441/1979 and Japanese Patent O.P.I. Publication No. 1-105738/1989 is disclosed a method for providing scratch and chemical resistance comprising adhering a polarizing film to a transparent plastic film through an adhesive and providing, on the resulting film, a resin film such as a polyester acrylate resin, a (meth)acrylate resin or an acrylurethane resin, which are then hardened. However, this method has problems in that the protective film is required and the polarizing plate itself is thick, resulting in increased weight.
Japanese Utility Model O.P.I. Publication No. 54-130441/1979 also discloses a method for enhancing scratch resistance employing evaporated SiO.sub.2 deposits, silicone type upsite and phenol resins, but the surface hardness or adhesion of the plastic plates is not fully satisfactory.
In Japanese Patent O.P.I. Publication No. 1-105738/1989 is disclosed a method for providing scratch and chemical resistance comprising providing on one side of an unsaponified triacetate film a hardened layer of a UW hardenable epoxyacrylate resin. However, in this method, the coating layer thickness increase, to enhance the surface hardness, is likely to cause cracks and adhesion between the polarizing plate and the coating layer deteriorates under high temperature and high humidity, since saponification can not be conducted.
In the above described polarizing plate, strong adhesion between the polarizing film and the protective film are required in order to obtain high reliability and durability under various circumstances. In order to enhance this adhesion, a polarizing film is used in which the surface of a cellulose triacetate film widely used for a protective layer is saponified with an alkaline solution and the cellulose triacetate film is provided on the polarizing surface through a polyvinyl alcohol type adhesive. However, since this method comprises employing a concentrated alkaline solution for the saponification, it is not favorable in view of manufacturing safety and environmental protection. Further, the alkaline processing causes bleed-out of the plasticizer or haze increase, resulting in poor quality.
An anti-static or hard coating treatment of the cellulose triacetate film is carried out before alkaline processing, but the successive alkaline processing reduces the effects of the treatment. Therefore, there is a problem in that various treatments of the cellulose triacetate protective film must be carried out after the alkaline processing.